Ironhead Diameter of inner adapter plate of XLCH

[IronHeadRon]

Edit, read below spec start timing is 15° btc not 10 and somebody measured more accurately than I So better numbers in comments than OP


I measured 2 and 1/16th making the radius 1 and 1/32nd

This means if you have proper timing mark (45° BTC) between adapter plates and want to retard it to the spec start timing (10° BTC) by moving your cam follower 17.5° in the direction of movement you can do so by moving your magneto clockwise such that the mark travels 17.5×pi/180×(1+1/32)"= ~0.314977388446" along the inner adapter plate's circumference, which is less than 1/256th" over 5/16"

Figured it'd be a neat tip for anyone who lost the pin or cable attachment thingy (control arm) like I did. Surprised to see so little of a turn, less than 3/8"!

I wonder two things

A can anyone confirm my measurement?

B my math checks right? A crank shaft change in position 35° retarted => cam shaft changes in position 17.5° retarded (I wanted to double it at first, but 720 crank = 360 cam so I think this is correct)
the rest is just basic formulas for the length of a theta radian sector of a radius r circle = theta×r and theta radians = theta degrees×pi/180

Once my rod covers come. I will mark it off and try it out, the goal is for my buck 40 ass to be able to kick her cold. Also plan on marking a good enriched starting point on idle needle. Will post pics when it happens, God willing this week.

[Captain Johnny]

Hey, aren't you the mathematics student? I remember a thread by you mentioning that.

Just set the timing and set the advance limit, swing it a little clockwise for retard then set that limit.

[needspeed]

I think you've got the math right.

If the mag's inner plate diameter is 2-1/16 I think you're correct that you would rotate the outer plate very close to 5/16 to retard the crankshaft timing 35°

But the retard timing for a 900 with advance timing of 45° is 15°BTC not 10. So you only need to retard it 30° not 35.

I've measured the inner plate at 2.028"

[IronHeadRon]

Originally Posted by needspeed

I think you've got the math right.

If the mag's inner plate diameter is 2-1/16 I think you're correct that you would rotate the outer plate very close to 5/16 to retard the crankshaft timing 35°


But the retard timing for a 900 with advance timing of 45° is 15°BTC not 10. So you only need to retard it 30° not 35.

I've measured the inner plate at 2.028"

Ty for checking, yes I have a BS in mathematics, helps me abstract models and problem solve but I'm neither an arithmetic wizard or engineer. So It's always good to double check.

I swear I saw ten somewhere in this book, but can't find it now, and I do see 15

so by your numbers we get 2.028/2*pi/180°*(30/2)° = .265464579228 less than half a thou under 17/64

Maybe I will just start at 1/4" and retard it as little at a time as I possibly can after that.

Any clue as to how they chose 15°? I get 45 was the best preforming but why 15 for best starting. In my head it seems like going at tdc or slightly after (net retarded) would be ideal. Maybe this is too wasteful to run (flame front not complete before valves open)? IDK.

But bottom line is, you brother, are correct.

[needspeed]

I'm not a math whiz by any means and I find the way you calculate this interesting and to be honest it's a bit over my head.

But I have fiqured stuff like this before. I start by finding the distance of 1 degree using the circumference and the fact that there are 360° in a circle.

So in this case:
2.028 x 3.14= 6.36792" circumference.
6.36792 / 360 =.0176" the distance of 1°
.0176 * 15 = .2653"

That's close to your answer. Maybe not as precise but close enough for what we're talking about. Just a different way.

I don't know how HD arrived at their retard specs, but it seems it's always 30° less than full advance.

So if 45 is advance 15 is retard.
On 1000 ironheads 40 is advance so 10 is retarded

[IronHeadRon]

Originally Posted by needspeed

I'm not a math whiz by any means and I find the way you calculate this interesting and to be honest it's a bit over my head.

But I have fiqured stuff like this before. I start by finding the distance of 1 degree using the circumference and the fact that there are 360° in a circle.

So in this case:
2.028 x 3.14= 6.36792" circumference.
6.36792 / 360 =.0176" the distance of 1°
.0176 * 15 = .2653"

That's close to your answer. Maybe not as precise but close enough for what we're talking about. Just a different way.

I don't know how HD arrived at their retard specs, but it seems it's always 30° less than full advance.

So if 45 is advance 15 is retard.
On 1000 ironheads 40 is advance so 10 is retarded

Your way is just as good, my calculator just has more significant figures for pi, but only the first three matter anyway's. But the math is the same
15×(one degree sector side length in a circle if radius 1)×(radius of circle) Radians, my way
=
15×(one degree sector length length in a circle of radius r) Straight to the circle at hand, your way

But I am glad the numbers check (both within 4thou of 17/64) I will update with pictures tomorrow and again if I find a non spec "sweet spot"

[bustert]

a lot goes into determining ignition start timing from mechanical to configuration spec like compression and such. in the auto world, if the timing is too adv, it is possible for the spark energy to jump to the adjacent tower post which will have less resistance than the one under compression.
another aspect is that if too adv, the possibility of kick-back is greater at cranking speeds and with a manual foot (been there done that) not a good feeling, even broken ankles can happen. in an engine with lower compression, it could use more adv since pressures are lower per given piston position(valves closed).
been too long, can barely remember what an hd mag looks like much less the insides.

[needspeed]

Originally Posted by IronHeadRon

..Maybe I will just start at 1/4" and retard it as little at a time as I possibly can after that. .

Another way to set the retard timing besides cleverly calculating the amount of mag rotation from the advance position is to just use the retard flywheel timing mark. Your '69 flywheels should have a double vertical line at 15° BTC. The single vertical line is the advance mark.

So just put the double line in the timing window on front cylinder compression stroke, rotate the mag till the points just break open and that's the retard setting.

[mikethebike61]

Old engineering joke...

The difference in an engineer and a mathematician.....

Have a beautiful woman sit in the exact middle of a park bench

place a mathematician at one end

place an engineer at the other end

the rules are you half-life the distance between you and the girl until you can kiss her....

the mathematician is upset as he loudly claims that this is an impossible contest because there will ALWAYS be a distance, however minute, between him and the girl

the engineer says....I'll get CLOSE ENOUGH

[IronHeadRon]

Originally Posted by mikethebike61

Old engineering joke...

The difference in an engineer and a mathematician.....

Have a beautiful woman sit in the exact middle of a park bench

place a mathematician at one end

place an engineer at the other end

the rules are you half-life the distance between you and the girl until you can kiss her....

the mathematician is upset as he loudly claims that this is an impossible contest because there will ALWAYS be a distance, however minute, between him and the girl

the engineer says....I'll get CLOSE ENOUGH

A mathematician, Physicist, and engineer are in a hotel and a fire starts in each room.
The engineer dumps buckets and buckets of water, putting out the fire
The Physicist calculates exactly how much water, to the drop, is needed
The mathematician sits up and proclaims "a solution exists" and goes back to sleep.


In school the running stereotype was engineers remembered formulas, mathematicians derived and critically thought out the system. But these days, we are a small bunch and I have more in common than in differences with any student of the sciences.